Field of the Invention
This invention relates to mono-focal objective lenses having the standard angle of view or a wider one, and, more particularly, to an objective lens, a portion of which is made axially movable for focusing.
Description of the Prior Art
Recently, the mainstream in the art of lens shutter cameras has adopted the auto-focus mechanism. The conventional objective lenses for this type of camera are not faster than 2.8 in F-number, and a lens having a large relative aperture of at least F/2.0 in the camera has not yet been developed.
The reason for this is that, since most of the existing faster lenses have been designed to be bodily movable for focusing purposes, the mass to be moved to effect focusing is so heavy that the auto-focus mechanism requires an electrical or mechanical driving torque which is much larger than what is available.
It is, therefore, desired to make the focusing provision not for the entire system but at only one group of the lens system.
It is already known in the art to provide a few examples of objectives having the latter focusing provision. For example, U.S. Pat. No. 3,020,804 discloses the use of a triplet type lens in combination with a focusing member in the form of a positive singlet of convex curvature toward the front. Because of that simplest form of the focusing member and the configuration of the front surface of the member to forward convexity, it is difficult to well correct the aberrations, and the range of variation of astigmatism during focusing is very large. And, particularly chromatic aberrations are deteriorated by that singlet lens. Another example of U.S. Pat. No. 2,503,789 is to perform focusing by a negative singlet, with its front surface of concave curvature toward the front, in the rear of a Tesser or Gauss type lens which is stationary during focusing. If this lens system were re-formed to a faster objective lens of relatively wide angle of view, its principal requirement for imaging performance would not compatible with the strengthening of the negative refractive power of the movable or last lens member. Therefore, the focusing member of so weak a power must be moved to a very long distance to cover the required focusing range. Since the height of incidence of the principal ray of the off-axis pencil on the movable lens member varies a large extent with focusing, the range of variation of astigmatism with focusing is largely increased.
Further, in both of the above-described examples, the paraxial ray is incideent on the movable member in strong converging action so that the height of incidence of the paraxial ray on the movable member also varies to a large extent with focusing. This results in a wide range of variation of spherical aberration with focusing, so that it is made difficult for the faster objective lens to preserve a sufficiently high grade imagery throughout the entire focusing range.
Such rear focusing provision is disclosed in U.S. Pat. Nos. 4,045,128 and 4,068,929. But they concern with the telephoto objectives which differ from the short focal length lenses in the method of balancing the various aberrations which determine the image quality. Accordingly, little suggestion can be derived from these telephoto objective lenses.